1. Field of the Invention
This invention relates broadly to carbon dioxide sequestration. More particularly, this invention relates to methods for reducing leakage of sequestered carbon dioxide.
2. State of the Art
Accumulating greenhouse gases have led to the advocation of separating and storing (sequestering) carbon dioxide from its sources. Carbon dioxide sequestration typically entails four distinct steps: the capture of the carbon dioxide from sources such as flue gas, transportation of the carbon dioxide to its ultimate storage site area, compression, and injection of the supercritical carbon dioxide downhole into the formation at a desired location (interval). Central to the success of the sequestration is the integrity of the downhole storage location; i.e., is the location sufficiently bounded by impermeable layers and seals. Because carbon dioxide is buoyant, particular attention is paid to the layer above the injected interval. For sequestration to be successful, any leak from the sequestration site must be inconsequential to inhabitants in the vicinity of the site. This is not exclusive to atmospheric leaks, but also to leaks into potable aquifers.
Generally, it is believed to be desirable to have more than one impermeable boundary between a selected injection (sequestration) zone and a potable aquifer. Thus, for example, assume with respect to FIG. 1 that a preferred injection zone is layer 0. Layer 0 is defined as being between z=0 and z=z0 where z is the vertical coordinate. Layer 0 is capped by a nearly impermeable layer 1 whose permeability is substantially smaller than layer 0 (preferably at least three orders of magnitude smaller) and is often in the range of ten or fewer microdarcies. Layer 1 is defined as being between z=z0 and z=z1. Above layer 1 is a permeable stratum, layer 2, which is defined as being between z=z1 and z=z2. Layer 2 in turn is assumed to be overlain by another nearly impermeable shale or shaly sand. With this arrangement, it would be generally assumed that a potable aquifer above layer 2 would be sufficiently protected.